Normal bone is constantly being remodeled, or broken down and rebuilt. Every week, humans recycle 5% to 7% of their bone mass. This remodeling process serves two primary functions: maintaining blood calcium levels and keeping the skeleton strong.
Two types of cells are involved in the remodeling of bone: osteoclasts and osteoblasts. Osteoclasts are the cells that break down bone, converting the calcium salts to a soluble form that passes easily into the blood. Osteoblasts produce the organic fibers on which calcium salts are deposited. In healthy young adults, the activities of these two cell types are balanced so that total bone mass remains constant.
Remodeling is a cyclic process that occurs at specific skeletal sites, with each remodeling cycle lasting about four months. Since bone resorption occurs rather quickly, most of the cycle is devoted to bone formation. The major event that triggers bone formation is the transition of mesenchymal stem cells into osteoblast cells. Osteoblasts deposit extracellular matrix (ECM) proteins to form the bone scaffold, which subsequently mineralize1.
The collagens that form fibrils are the most abundant components of most connective tissue and provide the three-dimensional scaffold that maintains tissue integrity2-4. Collagen type I is the major ECM protein of the bone, comprising 90% of its organic mass, and is highly expressed during bone formation5.
Collagen type I and other filbrillar collagens are first synthesized and secreted into the ECM in the form of soluble precursor, procollagen6. Proteolytic processing of procollagen N-propeptide and C-propeptide (PINP and PICP, respectively) by specific N- and C-proteinase lead to the production of mature collagen monomer capable of forming fibrils. PICP is cleaved by bone morphogenetic protein-1 (BMP-1)7. BMP-1 is a multi substrate enzyme, among its substrates are procollagen type I, II, III and other ECM precursors, such as lysyl oxidase, laminin 5, chordin, and probiglycan8,9. The cleavage of PICP by BMP-1 lowers procollagen solubility by at least a thousand fold and is critical for the self-assembly of collagen fibrils10. The rate of this processing appears to control fibril formation11. BMP-1 C-propeptide processing activity is regulated by another component of the ECM, PCPE12, 13.
In their search for bone formation markers, researchers have focused on metabolites and enzymes specific to osteoblast activity, mainly those involved in collagen type I synthesis and mineralization, but each was found to have disadvantages14, 15.
Current markers of bone formation include, among others, circulating procollagen type I C-propeptide (PICP), osteocalcin (OC), and bone-specific alkaline phosphatase (bone ALP). All of these reflect osteoblast activity during the process of bone formation and can be measured in serum15.
PICP is a soluble trimeric globular protein, produced simultaneously with the production of mature type I collagen molecules from procollagen, and released into the blood16. From the blood it is cleared by liver endothelial cells via the mannose receptor and has a short serum half life of 6-8 min. PICP may arise from other sources like tendons, skin, ligaments, cornea, and many interstitial connective tissues, but these non-skeletal tissues exhibit a slower turnover than bone, and contribute very little to the circulating propeptide pool. Different studies have shown good correlation between serum PICP levels and rate of bone formation but its clinical relevance is still viewed with skepticism17.
Osteocalcin is the most abundant non-collagenous protein of bone matrix. As opposed to PICP, osteocalcin is exclusively present in bone tissue, increasing significantly when skeletal growth is boosted. However its serum concentration has circadian variations, it is relatively unstable in serum samples and considerable inconsistencies have been reported among laboratories14. Other disadvantages include its release during bone resorption and rapid clearance by the kidney. In breast cancer patients with bone metastases, serum OC lacks diagnostic sensitivity compared to bone ALP, but in multiple myeloma patients, low levels of OC were found to be associated with severity of the disease and survival17.
Bone alkaline phosphatase (bone ALP) is an enzyme localized in the membrane of osteoblasts that is released into the circulation. The liver and the bone isoenzymes are the major contributors to the serum level of total ALP. Serum total ALP activity is the most commonly used marker of bone formation but it lacks specificity. Although most of the methods used to monitor the level of bone ALP have proved to be insensitive, nonspecific (as there was cross-reactivity with liver ALP) or technically complicated, in prostate cancer, bone ALP has been shown to be a more sensitive indicator than total ALP in the detection of bone metastases17. The precise function of the enzyme is yet unknown but it obviously plays an important role in osteoid formation and mineralization15.
U.S. Pat. No. 4,857,456 discloses an assay of BMP and anti-BMP antibody for the diagnosis of bone disorders, which may be carried out by comparing the BMP, anti-BMP antibody, or the ratio of the two to normal assay standards.
US 2003224501 discloses use of BMP polynucleotides, polypeptides, and antibodies for diagnostic use.
U.S. Pat. No. 6,037,139 teaches a system for assaying modulators of procollagen maturation, but does not teach the use of such modulators as biological markers.
U.S. Pat. No. 6,803,453 teaches use of antibodies associated with alterations in bone density. The compositions are useful in the diagnosis, prevention and/or treatment of diseases associated with a loss of bone density, such as osteoporosis.
WO 02/066962 teaches methods for determining cartilage degeneration or regeneration in a joint tissue in a patient by measuring levels of osteogenic protein-1 (OP-1 or BMP7) protein and/or mRNA in synovial fluid or joint tissue.
The background art teaches proteins related to BMP. However, BMP1 is an enzyme, while the remaining BMP proteins are not enzymes and have completely different roles. BMP1 is active post modelling for specific collagen deposition, while the others operate earlier in the process and affect bone cell activities. They are also related to non-bone activities, particularly BMP1. However, none of them are accurate, specific biochemical markers for bone formation which are sufficiently accurate and specific to be used as a single marker.
Bone morphogenetic proteins (BMPs) were first identified in fraction of demineralized bone extracts, and named for their ability to induce ectopic endochondral bone formation when implanted into the soft tissues of rodents18. BMP1 differed from the other BMPs members which are all members of the transforming growth factor (TGF)-β superfamily of growth factors, in possessing a distinct protein domain structure, that included a conserved protease domain. To date, at least 20 BMPs have been identified, some of which have been shown in vitro to stimulate the process of stem cell differentiation into osteoblasts in human and animal models. Having realized the osteoinductive properties of BMPs and having identified their genetic sequences, recombinant gene technology has been used to produce BMPs for clinical application—most commonly, as alternatives or adjuncts in the treatment of cases in which fracture healing is compromised. BMP-2 and BMP-7 are approved for clinical use in open fractures of long bones, non-unions and spinal fusion. However, despite significant evidence of their potential benefit to bone repair and regeneration in animal and preclinical studies, there is, to date, a dearth of convincing clinical trials19.
Fibrosis is a widespread pathological condition characterized by excessive extracellular matrix deposition, with type I collagen (the major fibrillar collagen) being the major component. This process leads to stiffness and loss of function of affected internal organs (e.g., heart, blood vessels, liver, kidney, lungs) and intense scarring of affected skin. Fibrosis can result from acute or chronic stimuli, including infections (viral hepatitis), alcoholism (leading to liver fibrosis/cirrhosis), autoimmune reactions (scleroderma; also known as systemic sclerosis or SSc) and mechanical injury (hypertensive heart disease, myocardial infarction). Nearly 45% of all deaths in the developed world are attributed to some type of chronic fibroproliferative diseases generally described as fibrosis20. Quantitation of fibrosis is important in order to assess both the progression of the disease and the therapeutic potential of new treatment protocols as mirrored by the regression of fibrosis. Because fibrosis typically progresses slowly, clinical trials to evaluate new treatment modalities could be long and expensive. Therefore, there is a desperate need to develop non-invasive diagnostic methods of fibrosis, in particular methods relying on a suitable serum marker(s), to quickly quantify changes in the natural history of the disease and assess the severity of the disease20.
For liver fibrosis, two classes of biomarkers have been identified. Class I markers reflect ECM turnover and fibrogenic cell changes and Class II biomarkers are based on algorithmic evaluation of common functional alterations of the liver that do not necessarily reflect ECM metabolism or fibrogenic cell changes21,22 
Biomarkers that specifically reflect collagen turnover have been proposed for monitoring both myocardial [reviewed in 23de Jong et al., 2011 and in 24López et al, 2010] and liver fibrosis [reviewed in 21Gressner et al., 2007]. Established serum biomarkers of collagen biosynthesis in cardiac and liver fibrosis are the C-propeptide of type I procollagen (PICP; also used as a marker of bone diseases) and the N-propeptide of type I and type III procollagens (PINP and PIIINP, respectively). Plasma markers of collagen degradation used to follow cardiac fibrosis include the C-terminal telopeptide of collagen type I (ICTP), matrix metalloproteinases (MMPs) 1, 2, 3 and 9, and tissue inhibitors of matrix metalloproteinases (TIMPs)23. A serum marker specific to cardiac fibrosis in hypertensive patients is brain natriuretic peptide (BNP)25. Together these markers are useful as predictive, prognostic or diagnostic tools.
A commercially available test for liver fibrosis, which is based on the measurement of several Class II biomarkers, called Fibrotest22, consists of an algorithm of five fibrosis markers, α2-macroglobulin, apolipoprotein A1, haptoglobin, γ-glutamyl transpeptidase (GGT), and bilirubin. Its performance is comparable to that of the ELF-test, which consists of an algorithm of 3 fibrosis markers of ECM metabolism, hyaluronic acid, PIIINP and TIMP-122; Friedrich-rust et al., 2010, hereby incorporated by reference)].